All walls and ceilings in a meat market shall be constructed of smooth, washable, non-absorbent material, free from cracks, crevices, and open joints, and shall be kept well painted. All right angles where walls and floors meet should be constructed with a rounded, sanitary joint so as to be easily cleaned. The walls and ceilings shall be cleaned frequently to prevent cobwebs and accumulations of dust, dirt, or other contaminating substances.